Objective: To describe in a 5-year-old Caucasian male with mitochondrial cytopathy, a biochemical growth hormone (GH) deficiency associated with normal GH biological activity as evaluated by Nb<sub>2</sub> cell bioassay and normal serum IGF-I and IGFBP3 values increasing slightly after GH administration. Method: Serum GH concentrations were measured with a commercial immunofluorometric assay and with a biological assay, which uses the Nb<sub>2</sub> cell line. Serum IGF-I and IGFBP3 concentrations were measured with RIA. Results: The GH-supplementary therapy was initially effective in terms of growth gain, but no therapeutic benefit was observed over a long period of time. Conclusion: In patients suffering from mitochondrial cytopathy, short stature seems to be attributed more to a disease-related inadequate protein substrate than to the non-classical GH deficiency.